The 12 full moons we see each year are named according to their relationship with the equinoxes and solstices. The names vary in different regions, but well-known examples are the harvest moon, which is the first full moon after the autumnal equinox, and the hunter’s moon, which is the second full moon after the autumnal equinox. Similarly the Lenten moon, the last full moon of winter, is always in Lent, and the egg moon (or the Easter moon, or paschal moon), which is the first full moon of spring, is always in the week before Easter.

By this system there are usually three full moons between an equinox and a solstice, or vice versa. However, because the lunar cycle is slightly too short for there to always be three full moons in this stretch of time, occasionally there are four full moons. When this happens, to ensure that the full moons continue to be named correctly with respect to the solstices and equinoxes, the third of the four full moons is called a blue moon.